FACTORS OF ORGANIC EVOLUTION. 473 



To my mind, all this is but the normal result of the divergence of 

 character, or the survival of the unlike. A new type finds places 

 of least conflict, it spreads rapidly and widely, and thereby varies 

 immensely. It is a generalized type, and therefore adapts itself at 

 once to many and changing conditions. A virile plant is introduced 

 into a country in which the same or similar plants are unknown, and 

 immediately it finds its opportunity and becomes a weed, by which we 

 mean that it spreads and thrives everywhere. Darwin and Gray long 

 ago elucidated this fact. The trilobites, spirifers, conifers, ginkgos, 

 were weed types of their time, the same as the composites are to-day. 

 They were stronger than their contemporaries, the same as our own 

 weeds are stronger than the cultivated plants with which they grow. 

 After a time the new types outran their opportunity, the remorseless 

 struggle for existence tightened in upon them, the intermediate unlike- 

 nesses had been blotted out, and finally only one or two types remained, 

 struggling on through the ages, but doomed to perish with the con- 

 tinuing changes of the earth. They became specialized and inelastic; 

 and the highly specialized is necessarily doomed to extinction. Such 

 remnants of a vanquished host remain to us in our single liriodendron, 

 the single ginkgo, and sassafras, and the depleted ranks of the conifers. 



My attention was first called to this line of thought by contemplating 

 upon the fact that cultivated plants difier widely in variability, and I 

 was struck by the fact that many of our most inextricably variable 

 groups — as the cucurbits, maize, citrus, and the great tribes of com- 

 posites — are still unknown in a fossil state, presumably because of their 

 recent origin. Many other variable genera, to be sure, are well repre- 

 sented in fossil species, as roses (although these are as late as the 

 Eocene), pyrus, prunus, and musa; but absolute age is not so signifi- 

 cant as the comparative age of the type, for types which originated very 

 far back may be yet in the comparative youth of their development. 

 The summary conclusions of a discussion of this subject were presented 

 to the American Association for the Advancement of Science two years 

 ago.^ A modification of these points, as I now understand them, would 

 run something as follows: 



1. There is a wide difference in variability in cultivated plants. Some 

 species vary enormously, and others very little. 



2. This variability is not correlated with age of cultivation, degree of 

 cultivation, or geographical distribution, 



3. Variability of cultivated plants must be largely influenced and 

 directed, therefore, by some antecedent causes. 



4. The chief antecedent factor in directing this variability is probably 

 the age of the type. New types in geologic time, are polymorphous; 

 old types are mononiorphous and are tending toward extinction. The 

 most flexible types of cultivated plants are such as have probably not 

 yet passed their zenith, as the cucurbits, composites, begonias, and the 



' Proc. A. A. A. S. 1894, 255 ; Botanical Gazette, XIX, 381. 



